The present invention broadly relates to access control equipment, and more particularly to electronic equipment designed to restrict unauthorised access to stored or transmitted data or to secure premises.
The control of such access is frequently afforded by the use of an interrogation unit and a transponder. At its simplest, the interrogation unit transmits an enquiry signal which may be received by a transponder within its range. The transponder is designed to modify the received signal in a predetermined manner and then retransmits the modified signal for reception by the interrogation unit. The modifying of the signal may be such as simply to distinguish between any "authorized" transponder and an unauthorized one or may be characteristic of the specific transponder alone, in which latter case the modified signal received by the interrogation unit identifies that transponder and no other.
Thus, if the transponder is carried by a person, vehicle, animal or piece of equipment then the interrogation unit is enabled to permit or reject access of a specific person or vehicle to controlled premises or to control access of a person or equipment to stored data. For example, when the equipment is capable of receiving transmitted data, it may be permitted to do so only when it has been identified or authorized by a correctly-responding transponder.
Many systems for operating such equipment are known. For example, the interrogation unit may permanently or at intervals create a signal or field that will cause any transponder within range to identify itself. In a second system, any transponder coming within range will be caused to transmit an identifying signal to the interrogation unit. In a further alternative a transmission is initiated from the transponder and the interrogation unit processes the information so conveyed.
In such systems there is generally no problem in making a positive identification of a particular transponder, so long as that transponder is the only one within range of the interrogation unit, or the only one signalling the interrogation unit at a given time. Unless one of these criteria is met, then there can be difficulties in resolving the signals from the responders.
A further difficulty arises when the system protocol is such as to cause a transponder to transmit its same identifying signal more than once when traversing any interrogation field. Under these circumstances, the signal could be monitored by an unauthorised person, and thereafter reproduced in order to gain illicit access.
The present invention seeks to avoid these difficulties.